Broadband internet has experienced a compounded annual bandwidth growth rate exceeding 30% over the last decade. The momentum shows no sign of slowing down as wireless broadband, due to the smart phones and portable devices, joins the game. Fibers that facilitate much of such land-line bandwidth supports in the past now challenge device and equipment development to keep up the speed to fill their available bandwidth capacities. In core networks, transport equipment can now support 40 Gbps and 100 Gbps per wavelength using dense wavelength division multiplexing (DWDM) carriers. 400 Gbps and even 1 Tbps per wavelength channel are being discussed and laboratory tried.
The core network availability of such high capacity data inevitably puts pressure on fiber optical transceivers to keep up with the bandwidth growth so that the Internet routers and switches can be efficiently used. IEEE has released some international standards on 40 Gbps and 100 Gbps Ethernet based transceivers. For the first time in history, we have seen that in order to keep up with the bandwidth growth demand of transceivers, more than a single wavelength are called for duty in order to transmit 40 or 100 Gbps data over 10 km distance via a single mode (SM) fiber. This is largely because even by using various data compression and forward error correction (FEC) techniques, no compact and right power consumed electro-optic modulator can handle such a high bandwidth data single handedly. 4×10 Gbps and 4×25 Gbps formats have been proposed to provide the aggregate data rate of 40 Gbps and 100 Gbps using 4 wavelength channels for transceivers.
After successful developments of 2.5 Gbps and 10 Gbps single wavelength fiber optic transceivers, the transceiver makers have been getting used to the device form factors, such as small form pluggable (SFP) and quad small form pluggable (QSFP), each of which typically has an electrically pluggable connector at back- and duplex fiber (2 fibers each for input and output) connector adapter port at front-end. Inside the SFP, each of the so-called transmitting optical sub-assembly (TOSA) and receiving optical sub-assembly (ROSA) unit is residing on a printed-circuit board (PCB) that provides power, control and various other supporting functions. While a TOSA typically has a laser and a modulator along with some coupling optics to a fiber output, a ROSA has a photo-detector and a trans-impedance amplifier along with some coupling optics for a fiber input.
In prior art, a typical fiber optical transceiver architecture is depicted in FIG. 1. Traditional TOSA units and ROSA units are very compact and deal with a single wavelength with no multiplexing (Mux) and demultiplexing (DeMux) functionalities needed. To embrace the new standards, the transceiver and TOSA/ROSA makers now are working on integrating WDM Mux/DeMux capabilities in order to offer the complete 40 Gbps and 100 Gbps transceiver capabilities.
FIG. 2 shows a 4-ch TOSA/ROSA. It has a fiber port and a coupling lens that collimates an optical beam from an incoming fiber. In a zig-zag free space geometry, the collimated beam passes through 4 WDM thin-film filters (some reflected by the mirrors before reaching their destination filters). In a ROSA case, each such a beam is refocused back by its micro lens to one of four photo-detectors. On the contrary, for a TOSA case, optical signals are first generated by the four WDM lasers, through their respective microlenses to be collimated and then traverse through the zig-zag cavity made by the 4 thin-film filters and 3 mirrors that combine these four laser beams into one single beam (hence multiplexing) before being focused into the fiber port. For simplicity, no other electronic part details are shown in FIG. 2. One fundamental advantage of this thin-film based Mux/DeMux scheme is that the light power loss is small. Typically the maximum loss will be practically smaller than 1.25 dB.
Another prior art that does not use thin-film filter as WDM mux-demux mechanism works with a planar lightwave circuit (PLC). Shown in FIG. 3 is a typical PLC based combiner that acts as a 4 to 1 WDM input combiner. There are no filters or mirrors employed but the price to pay for the simplicity is that any of the laser input will have to suffer at least 75% (or 6 dB) minimum power loss due to basic physics constraints of combing SM optical power using a splitter/combiner or non-multiplexing scheme. The PLC can also be used to make a WDM multiplexer. The advantage of using a PLC chip with butt coupling directly to a laser or photodetector chip on one end and to a fiber on the other end is its relative simplicity. The PLC is a mass-production friendly technology. However, the limitation is the high power coupling loss, not only just for the >6 dB fundamental combing loss in the PLC combiner case, but also for the WDM Mux/DeMux scheme that also suffers severe waveguide bending loss associated with the high refractive index material for PLC waveguides. For example, using a silicon material for the PLC, the butting oriented coupling at both front and back ends of the silicon PLC chip together with its inherent waveguide absorption and bending loss will mount to be >5 dB. Silica may be used and has a matching refractive index to glass fiber to form PLC waveguides on chip. However, such a chip will inevitably have much larger chip area unless a chip designer plans to endure severe bending loss (e.g., >5 dB if making the chip as compact as its silicon counterparts).
FIG. 4 shows a parallel optics transceiver for 40 Gbps for multi-mode (MM) fibers based on vertical cavity surface emitting laser (VCSEL) array technology. VCSEL is directly modulatable to ˜10 Gbps and very inexpensive comparing to distributed feedback (DFB) lasers, thus suitable for various short reach links (e.g., <300 meters). Many applications can afford using parallel ribbon fibers (MM fibers) to link between some equipment racks inside a data or cloud center). FIG. 4 shows that TOSA and ROSA for the VCSEL based transceiver have 4 fiber ports. Such TOSA and ROSA due to not having Mux/DeMux components on board (see FIG. 5) are much compact and straightforward than their SM counterpart shown in FIG. 2. As far as the current transceiver types are concerned, QSFP, XFP, CXP, CFP, CFP-2 form factor standards can support array fiber connector interfaces such as MPO or MTP connectors that allow up to 12 fibers in a single row on a multi-fiber ferrule. FIG. 6 shows a typical MPO/MTP array fiber connector. An exemplary QSFP optical transceiver accepts this array fiber connector format. An MPO/MTP connector ensures all fibers to be connected with a small ˜0.35 dB insertion loss because it uses a high precision guide pin structure on the two ends of the rectangular shaped fiber ferrule.
In a brief summary, the above mentioned prior arts illustrate several important facts:                Transceivers must incorporate multi-wavelengths in the future for the era where the data rate needs to be in excess of 40 Gbps for a SM fiber reach of >2 km.        Many current transceiver standards shall be capable of accepting MTP/MPO fiber array connectors.        Incorporating multi-wavelength Mux/DeMux inside transceivers will cost the already tight real estate inside a transceiver regardless of using either thin-film or PLC based Mux/DeMux technology.        
Accordingly, there is a need for improved techniques that separate the multi-wavelength Mux/DeMux functionality out of the main transceiver body. This separation allows the TOSA and ROSA community to continue to focus on active component (e.g., lasers, modulators, photo-detectors) integration management which itself sees increasing complexity that demands more space inside a transceiver. The separated MDOC is a purely passive component, thus the makers thereof in future can focus on making this type of passive devices more compact, reliable and lower in cost.